High-mobility amorphous silicon displaying non-dispersive transport properties

Abstract

The present invention teaches a combination of parameters for the glow discharge decomposition of silane deposition of an amorphous silicon semiconductor having non-dispersive high mobility transport of majority carriers through the semiconductor material, useful in switching devices such as diodes, transistors and the like.

What is claimed is: 
     
       1. A method for producing an amorphous silicon semiconductor having nondispersive transport of majority carriers and further having a majority carrier mobility higher than about 0.6 cm 2  /volt-second, said method comprising the steps of: providing a substrate for deposition of said silicon thereon;   heating said substrate to a temperature between about 220° C. and about 350° C. in a vacuum chamber;   providing a partial pressure of silane in said chamber between about 800 millitorr and 850 millitorr;   sustaining a direct current glow discharge in the vicinity of said heated substrate at a current density between about 0.05 mA/cm 2  and about 0.3 mA/cm 2  for a period of time sufficient to deposit a layer of an amorphous silicon semiconductor on said substrate.   
     
     
       2. A method for producing an improved amorphous silicon semiconductor switching device comprising the steps of: providing a substrate for the deposition of said amorphous silicon thereon; heating said substrate to a temperature between about 220° C. and about 350° C. in a vacuum chamber;   providing a partial pressure of silane in said chamber between 800 millitorr and 850 millitorr;   sustaining a direct current glow discharge at a current density between about 0.05 mA/cm 2  and 0.3 mA/cm 2  in the vicinity of said substrate for a period of time sufficient to deposit a layer of amorphous silicon thereupon;   forming a diode junction comprising said amorphous silicon layer whereupon majority carriers injected into or generated within said semiconductor are nondispersively transported through said material under the influence of an electric field at a mobility above about 0.6 cm 2  /volt-second, increasing the switching speed of said device.   
     
     
       3. The method set forth in claims 1 or 2 wherein said partial pressure of silane is maintained between about 800 millitorr and about 850 millitorr by providing a feed rate of gaseous silane of about 10 standard cm 3  /min and concurrently evacuating said chamber at a rate sufficient to maintain said pressure. 
     
     
       4. The method set forth in claim 3 wherein said glow discharge is sustained by providing a voltage between a generally cylindrical anode and cathode electrode situated in said evacuated chamber, said voltage being between about 700 volts to about 800 volts for a cathode to anode spacing of about 2.5 cm, said anode or cathode having a diameter about equal to about 7.6 cm. 
     
     
       5. The method of claim 4 wherein said glow discharge is sustained for a period of between about 5 min. to 20 min. to deposit an amorphous silicon layer greater than 1 micron in thickness. 
     
     
       6. A method for producing an improved amorphous silicon semiconductor having nondispersive majority carrier mobilities higher than about 0.9 cm 2  /v-sec, said method comprising the steps of: providing a substrate for the deposition of said amorphous silicon thereupon;   heating said substrate to a temperature between about 220° C. and about 350° C. in a vacuum chamber previously evacuated to a pressure below about 10 -5  torr;   providing a partial pressure of silane in said chamber, said partial pressure of silane being between about 30 millitorr and about 50 millitorr;   sustaining a radio frequency glow discharge having an input power between about 0.2 watts/cm 2  and about 0.4 watts/cm 2  coupled to at least said silane by means of at least two electrodes, said discharge being sustained in a vicinity of said heated substrate and for a time sufficient to deposit a layer of amorphous silicon.   
     
     
       7. A method for producing an improved amorphous silicon semiconductor switching device having non-dispersive electron mobilities greater than about 0.9 cm 2  /volt-second, said device constructed by the process comprising: providing a substrate for the deposition of amorphous silicon thereon;   heating said substrate to a temperature between about 220° C. and about 350° C.;   providing a partial pressure between about 30 millitorr and about 50 millitorr of silane in a vacuum chamber previously evacuated to a pressure below about 10 -5  torr; sustaining a radio frequency glow discharge at a power between about 0.2 watts/cm 2  and 0.4 watts/cm 2  coupled to at least said silane by means of at least two electrodes, said discharge being sustained in a vicinity of said substrate for a period of time sufficient to deposit a layer of amorphous silicon thereupon;   forming a diode junction comprising said amorphous silicon layer whereupon majority carriers injected into or generated within said semiconductor are non-dispersively transported through said material under the influence of an electric field at a mobility above about 0.9 cm 2  /volt-second, increasing the switching speed of said device.   
     
     
       8. The method set forth in claims 6 or 7 wherein said electrodes comprise two generally cylindrical electrodes spaced in generally parallel relationship having an inter-electrode spacing of about 2 inches and each having a mean diameter of about 8 inches. 
     
     
       9. The method set forth in claim 8 wherein one of said electrodes is a cathode and said heated substrate is held contiguous to said cathode electrode. 
     
     
       10. The method of claim 9 wherein said glow discharge is sustained at a R.F. power frequency about equal to 13.56 megahertz. 
     
     
       11. The method of claim 8 wherein a relationship between a flow rate of silane input to said chamber and said electrodes is expressed as a gas feed rate of about 0.05 cm 3  /min to about 0.06 cm 3  /min per cm 2  of electrode area, said electrode area being defined as a surface area of one of said two electrodes and said surface comprising an interelectrode facing surface.